Conventional image intensifier systems in clinical use typically comprise the major elements of an x-ray source and a target both supported on a rigid and relatively massive structure that maintains them substantially at 180 degrees to one another. The x-ray source and target are spaced apart by sufficient distance to allow part or all of the patient to be interposed between them. The support is generally referred to as a `C` arm. The system also includes a monitor screen that displays the image of the body part lying in the x-ray beam.
X-rays and the machines that generate them are hazardous, not only to patients but also to those working with or near them and it is well established that cumulative doses can cause cancer, cataracts and gonadal damage. Operating theatre and radiographic department staff at risk wear lead aprons to protect them from exposure but in some procedures, especially in the field of orthopaedics, the surgeon in particular may have to take a calculated risk and knowingly expose himself to x-radiation.
This practice has increased in recent years with the introduction of reliable intramedullary devices for the fixation of fractures of long bones. Although these devices have led to substantial improvements in fracture management, the techniques for their satisfactory implantation are somewhat demanding.
For example, one device that is increasingly widely used in femoral and tibial fractures is the Grosse-Kempf intramedullary nail. This is a metal tubular structure, with a single slit down its long axis. The diameter of the nail is such that it may be accommodated within the intramedullary canal of the bone. Under general anaesthesia the nail is introduced into the surgically exposed upper end of the proximal bone fragment. The nail is then driven carefully down its length. The fracture is reduced by manipulation of the distal fragment using an image intensifier to ensure that the fragments are correctly aligned. The nail is then driven down into the distal fracture fragment. In order to increase the rigidity of the fixation, transverse locking screws are used at each end of the nail which has holes to accommodate them. Because the nail is a tubular structure the term `nail hole` in this context really means a front hole and a back hole in the tube. Fixation of the end of the implant at the proximal end, close to the entry site, involves the use of a jig to locate the hole on each side of the nail. This is relatively straightforward because the bone is exposed. However, the holes at the distal end of the nail lie within a part of the bone that is not exposed and can only be located by using x-rays, in particular an image intensifier. It is important to note that the positions of the lower or distal holes cannot be found by direct measurement with reference to the proximal jig. This is because the nail has inherent axial and rotational flexibility; it is not a straight rigid structure and will often twist during insertion.
Although Kempf et al (1985; Closed locked intramedullary nailing (J. Bone Joint Surg. 67A, 709) have described a targeting device which is mounted on the image intensifier, this is difficult to master and the majority of surgeons prefers to use a freehand technique. With this method, in the first stage, the image intensifier is moved to the general area of the limb where the selected nail hole is believed to lie and is then switched on. The `C` arm and the limb, if necessary, are then manoeuvred until the nail hole is located. This is evident when its image appears upon the monitor screen. Correct alignment is achieved when images of both the front hole and the back hole appear as concentric circles in the middle of the monitor screen.
With the drill held obliquely, the surgeon moves the drill bit over the soil tissue on the surface of the limb to where he estimates the point of entry will be. The x-ray beam is then switched on briefly, during which time at least the surgeon's hand and forearm are exposed to the `live` radiation. Without moving the drill bit, the surgeon now refers to the monitor screen in order to check the position of the drill bit image relative to the concentric images of the nail hole. If his first guess at the entry point was correct he may choose to mark the limb. This is often done by making a small cruciform stab incision through the skin, sometimes extending down to the periosteum. More often than not the surgeon will need to make at least one and maybe several further screenings in order to position the drill bit accurately, on the soft tissue, over the bone to be drilled. Each screening represents an exposure for the surgeon since it is generally accepted that significant radiation is delivered to objects within 80 cms radially from the axis of an x-ray beam.
Whether he chooses to mark the proposed entry point or not, the surgeon must now make another educated guess regarding how to position the drill in order to make his entry into the bone and engage the nail hole cleanly. This is done by visual reference to the source cone and the target of the inactive image intensifier, the aim being to align the drill accurately with the beam axis which is, of course, invisible.
As will by now be apparent, this entire procedure is difficult, even for the experienced surgeon. It is common for the nail hole to be missed during the first attempt at drilling and this generally necessitates repositioning the drill bit with the x-ray beam on, as before. In addition a drilling `miss` almost invariably involves some damage to bone.
One of us (J. G.) has collected data which suggests that in the U.S.A. the average x-ray exposure time for a surgeon during an intramedullary nailing procedure probably exceeds 3 minutes whereas in the United Kingdom the exposure time is nearer to 7 minutes.
At this stage we do not have sufficient data to postulate reasons for the difference but it is disturbing that these timings are occurring on recent machines with stored image facilities so that exposure times are already shortened in comparison to those which would occur if images could not be stored. What is clear is that, if intramedullary nailing remains popular during the next few decades, a surgeon aged thirty when he takes up the technique and carrying out one procedure per week, could receive a cumulative dose of x-rays equivalent to over 150 hours continuous exposure. This is a frightening prospect and one that should concern all surgeons carrying out these procedures.
The orthopaedic establishment is now aware of the dangers. In an editorial article in the Journal of Bone and Joint Surgery in May 1992, Hynes et al, wrote "The recent upward revision of risk estimates should serve as a timely warning to all occupationally-exposed radiation workers, including orthopaedic surgeons, that there continues to be uncertainty in predicting the effects of low dose radiation, and that it is wise to act on the basis that there is no safe dose of radiation." (Ionising radiation and the orthopaedic surgeon. J. Bone Joint Surg [Br] 1992: 74B: 332-4).
Accordingly, we have invented and developed a device that substantially reduces this risk and which is in line with current attitudes and accepted techniques for preserving the personal safety of those working in the operating room, those operating x-ray apparatus and patients exposed to this form of ionising radiation. The instant invention is also in accordance with the `Alan (As Low As Reasonably Achievable) Principle`(Euratom 1980).
A single low power laser beam, suitably aligned and positioned so as to be co-axial with the x-ray beam of an image intensifier system, would also lie along the axis of an intended drill hole in a bone, once the limb had been correctly positioned. This would enable accurate alignment of the drill to the axis of the x-ray beam to be achieved by reference to the laser light and without additional reference to x-ray screening. This would significantly eliminate direct exposure of the surgeon. Until now, means for achieving this have presented significant mass in the beam path with consequent degradation of the image. This is not a great problem with patients who have good quality bone because they generally yield clear images, however, in older and osteoporotic patients the images are often rather unclear and any significant degradation is unacceptable.